FIG. 12 is a cross-sectional view showing one example of a general configuration of a color cathode ray tube. As shown in FIG. 12, a color selection electrode (shadow mask) 3, a magnetic shield 1 for reducing the effect of the geomagnetism on tracks of electron beams 5, and a frame 2 for supporting the shadow mask 3 and the magnetic shield 1 are contained in an evacuated glass container formed of a panel 6 and a funnel 7. An electron gun 9 is contained in a neck portion of the funnel 7. Electron beams 5 emitted from the electron gun 9 are deflected by a deflection yoke 8 so that they pass through slot-shaped apertures formed on the shadow mask 3 and scan a rectangular phosphor screen 4 formed on the inner face of the panel 6.
For convenience in the following explanation, as shown in FIG. 12, an XYZ-three dimensional rectangular coordinate system is defined, in which the X-axis is a horizontal axis perpendicular to the tube axis, the Y-axis is a vertical axis perpendicular to the tube axis, and the Z-axis is the tube axis. The X-axis and the Y-axis intersect with each other on the tube axis (Z-axis).
Conventionally, it has been pointed out that the problem of halation is inherent in the cathode ray tube having the above configuration. Halation is a phenomenon caused by an electron beam 5 that should enter the shadow mask 3 directly but actually enters the shadow mask 3 after being reflected by the frame 2 or the like due to overscan or the like when it is deflected to the periphery of the screen. Such an electron beam 5 then reaches the phosphor screen 4 to cause the screen to emit light, resulting in degraded contrast.
As a solution to this problem, JP 2(1990)-244542 A discloses bending a tube-axis-side end portion of a frame 2 having a substantially L-shaped cross section toward a panel 6 to provide a bent end portion 12, as shown in FIG. 13. According to this configuration, halation is prevented because an overscanned electron beam 5 strikes the inclined face of the bent end portion 12 and is reflected toward the side opposite to the phosphor screen 4 side.
Further, JP 11(1999)-120932 A discloses forming a number of recesses on an inner surface of a skirt portion 13, which is a portion to be joined with an inner face of the frame 2, of a shadow mask 3. According to this configuration, halation is prevented because an overscanned electron beam entering the inner surface of the skirt portion 13 is scattered.
Furthermore, JP 5(1993)-314919 A discloses forming a bent portion by bending a corner portion of a magnetic shield 1 provided at its end portion on the frame 2 side toward the tube axis so as to be substantially perpendicular to the tube axis. According to this configuration, halation is prevented because an overscanned electron beam is shielded by the bent portion and thus cannot reach the screen.
However, the inventors of the present invention have found the following fact through experiments. In a cathode ray tube with a total deflection angle of 115° or more, as shown in FIG. 14A, an electron beam 5 is reflected not only by the frame 2 having a thickness of about 1.8 mm but also by the end face (the face opposing the tube axis) of the magnetic shield 1 having a thickness of only about 0.15 mm. As a result, a linear halation pattern formed of a number of red, green, and blue vertical lines arranged repeatedly appears on the right and left sides of the screen.
The cause of such halation is considered to be as follows.
In a cathode ray tube with a normal deflection angle, as shown in FIG. 13, an electron beam 5 entering and reflected from the end face 11 of the magnetic shield 1 is reflected toward the side opposite to the phosphor screen 4 side by the frame 2 and thus causes no halation. However, in a cathode ray tube with a total deflection angle of 115° or more, an electron beam 5 enters the end face (the face opposing the tube axis) 11 of the magnetic shield 1 at a smaller incident angle as shown in FIG. 14B, which shows an enlarged view of the portion XIV B, the vicinity of the end face of the magnetic shield 1, shown in FIG. 14A. Thus, while an electron beam 5a entering and reflected from the region near the frame 2 in the end face 11 is reflected by the frame 2 similarly to the electron beam shown in FIG. 13, an electron beam 5b entering and reflected from the region apart from the frame 2 in the end face 11 does not strike the frame 2 and thus is allowed to reach the screen. Besides, the end face 11 has a poor flatness, which causes the above-mentioned linear halation pattern having high visibility to appear in a particular portion of the screen, unlike the conventional halation pattern causing the entire screen to emit light uniformly.
It is apparent from FIGS. 14A and 14B that the bent end portion 12 provided at the edge of the frame 2 as disclosed in JP 2(1990)-244542 A is not effective in preventing such halation occurring in a cathode ray tube with a large deflection angle.
Further, in a cathode ray tube with a large deflection angle, a track of an electron beam 5 entering a corner portion of the screen 4 forms a small angle with the screen 4. Therefore, if the bent portion as disclosed in JP 5(1993)-314919 A is used to shield an overscanned electron beam, an electron beam for forming an image also is shielded, which brings about a problem that a shadow appears on the screen.
By making the distance between the end face 11 of the magnetic shield 1 and the tube axis longer (i.e., by increasing the amount that the end face 11 is recessed from the edge of the frame 2 on the tube axis side), it becomes possible to shield an electron beam reflected from the end face 11 by the frame 2. However, this results in reduction in area of the bent portion, which is provided on the screen 4 side of the magnetic shield 1 and is substantially perpendicular to the tube axis, and thus brings about the problems such as degraded magnetic shielding effect, degraded stability in fixing the magnetic shield 1 to the frame 2, and the like.
On the other hand, as a measure against halation in a cathode ray tube with a small deflection angle of 115° or less, it is difficult to apply the method proposed in JP 2(1990)-244542 A to a cathode ray tube of a so-called tension-mask type, in which a shadow mask is stretched while being provided with a tensile force, because the degree of freedom in the shape of the frame is limited in such a cathode ray tube. Further, the method proposed in JP 11(1999)-120932 A requires processing the inner surface of the shadow mask, resulting in high cost. Besides, this method is not applicable to a cathode ray tube of a tension-mask type. Furthermore, the method proposed in JP 5(1993)-314919 A does not provide any shielding effect on an electron beam passing through the portion other than the corner portion.